Reduction of chromium oxides



REDUCTION OF CHROMIUM OXIDES Hendrik de W. Erasmus, Lewiston, N. Y., assignor to Union Carbide Corporation, a corporation of New York 1 No Drawing. Application August 19, 1955 Serial No. 529,577

6 Claims. (Cl. 75-84) The invention relates to the solid phase reduction of chromium oxides by carbon and more particularly to the production of sintered shapes of low-carbon chromium metal alone or in association with other metallic elements.

Much prior work has been done on thereduction of chromium oxides by carbonaceous materials such as chromium carbides and pure carbon. Extensive investigations on this subject have been conducted by the United States Bureau of Mines.

The principal reversible reaction occurring in the production of low-carbon chromium metal from chromic oxide by carbon reduction is To those skilled in the art, it is well known that decreasing the partial pressure of the carbon monoxide causes the reaction to proceed toward the right. According to the Bureau of Mines, Technical Paper 662, page 36, the calculated equilibrium carbon monoxide partial pressure of this reaction is 2.87 10 atmosphere at 1407 C.; but this equilibrium partial pressure decreases as the concentration and accompanying activity of the carbide and oxide in th'ereac'ting mixture decrease. Also, according to the Bureau of Mines Bulletin 383, the calculated vapor pressure of chromium is 10- atmosphere at 1410 C., and 1() atmosphere at 1594 C. Thus, the temperature and absolute pressure at which complete decarburization can take place approach those at which chromium metal begins to evaporate or at which the reacting mixture melts, particularly since the inside of a briquet reacts more slowly than its surface layers and those briquets which are closest to the source of heat become hotter. When the reacting mixture melts, the liquid boils and splashes due to the evolution of gas unless the carbon and oxygen concentrations have reached sufiiciently low values to effect the maintenance of reasonably quiescent molten metal under vacuum. However, if the reaction is carried out in the solid state the time for its completion is greatly extended as the carbon and oxygen contents of the reactants must diffuse toward each other from separate solid particles. Given enough time and adequate contact, this can happen, but the cost is prohibitive and the process tedious. Accordingly, solid phase reactions between chromic oxide and carbon have not achieved industrial acceptance in competition with processes of chromium production such as electrolysis of dissolved chromium compounds, or the reduction of chromic oxide by aluminum, silicon or chromium silicide.

The present invention is based upon the discovery that the total heating time required for the complete reduction of chromic oxide by carbon in the solid state may be greatly shortened by preliminarily heating pellets composed of a stoichiometrical mixture of comminuted reacting materials, in vacuo or even at atmospheric pressure,

until there is a marked decrease in the rate of gas evolution, and thereafter cooling, regrinding, pelleting and 2,833,645 Patented May 6, 195 8 reheating in vacuo the reacting materials to obtain com plete reduction of the chromic oxide.

In the solid phase production of chromium metal according to the invention, chromic oxide and the selected carbonaceous material are first comminuted and thoroughly blended as by ball milling. The amount of carbonaceous material should be such that the carbon therein is approximately the quantity required stoichiometrically to effect reduction of the chromic oxide.

' After mixing, the charge is preferably pelleted or otherwise converted into compressed lump form. Pelleting of the charge provides excellent contact between reacting particles and also the necessary interstitial space for the escape of reaction gases as well as a convenient form for handling the material. Bonding agents such as chromic acid and water, or molasses may be advantageously employed in the formation of pellets. The pellets may then be charged into any suitable vacuum furnace and heated, preferably but not necessarily, in vacuo or under reduced pressure at a temperature at Which the reaction proceeds but below the melting point of the pellets until there is a marked decrease in the volume of evolved gas from the reacting materials. At this pointthe pellets may be cooled to about 200 C. or below and discharged from the furnace. This primary heating procedure ordinarily will remove or more of the combined oxygen in the oxide constituent of the pellets and leave the pellets in a condition suitable for remilling, reblending and repelleting prior to the second stage heating operation.

At the end of the first stage heating and after a preliminary remilling, it may be desirable to adjust the composition to obtain a stoichiometrical balance between the carbon and oxygen in the mixture. This operation should be followed by careful blending as the simple localized addition of carbon or oxide to efiect the desired balance will not work satisfactorily. Such an addition, without careful blending, has the eifect of adding one reagent as localized particles of high concentration to a much larger mass in which the other reagent occurs in low concentration so that the reagents have to diffuse through the distance of several particle diameters in order to contact each other with a consequent slowing down of the rate of reaction. It is preferable to use as a blending agent, other batches of the first stage of reaction in which the departure from the stoichiornetric ratio of carbon and oxygen is in the opposite direction.

Following the blending step, the charge is again pelleted and the pellets heated in vacuo or under reduced pressure at a temperature at which the reaction proceeds but below the melting point of the pellets to ei lect the final reduction of the chromic oxide to chromium metal.

Both the primary heating procedure and the final heating should be at temperatures above 1100 C. and below the melting point of the constituents of the pellets. A maximum temperature of 1600" C, has been found to be satisfactory. Preferably the pellets are heated to a term perature of between 1250 C. and 1400 C. The heating, as indicated above, continues until the rate of evolu tion of carbon monoxide gas decreases markedly.

The pellets of chromium metal made according to the invention are in the form of highly compacted, uniformly porous, strongly cohesive, non-friable sintered aggregates and are preferably of a size not exceeding about two inches in any dimension. The pellets have an apparent density of about 5 to 5.5 grams per cubic centimeter as compared with a density of 6.92 grams per cubic centimeter for solid chromium metal.

Nickel, cobalt and other non-volatile metals may be introduced into the final product by incorporating these metals or their oxides or carbides into the original mixture in such amounts as to maintain a stoichiometrical relationship between carbon and oxygen in thetotal mixtureto be treated.

Pellets of chromium metal made according to the invention are particularly suitable for use in the production of chromium-containing ferrous and non-ferrous alloys and chromium steels. The introduction of a given amount of chromium into a molten bath of metal may be accomplished more rapidly with the pellets of the invention than with a corresponding amount of similarly sized pieces of solid chromium metal. The shorter production time obtained from their use enables a savings in manhours and a corresponding increase in plant production for given furnace facilities. In addition, there is less loss of chromium due to oxidation at the high temperatures employed in the alloying processes.

The following specific example will serve to illustrate the manner in which the two-stage, solid phase method of the invention is applied to the production of sintered chromium metal pellets from chromic oxide by carbon reduction.

The chromic oxide employed had the following analysrs:

Percent Cr O 98.34 F2O3 C 0.05 S 0.1 l Si 0.06 The carbon black employed had the following analysis: Percent Fixed C 98.62 Volatile matter 1.38 Ash Nil A stoichiometric mixture of chromic oxide and carbon black was prepared and ground together in a ball mill for about 15 minutes; 4% chromic acid, by weight in the form of a water solution was then added as a binder and the material was then mechanically mixed for about one hour. Pellets were prepared from the moist mixture in a high pressure pelleting press and the pellets so made were stacked in pans to a depth of about 8 inches and dried overnight at about 150 C.

After drying the pans of pellets werecharged into a vacuum furnace. The pressure in the furnace was maintained at 2.5 mm. of mercury by manipulating the heating schedule up to a maximum temperature of 1250 C. The temperature wasmaintained at 1250 C. until the furnace pressure dropped to 500 microns of mercury after which the furnace was cooled to 200 C. with argon and discharged. This procedure removed 85 to 90% of the oxygen in the charge as carbon monoxide and left the first-stage product in a coky" condition suitable for remilling in preparation of the second-stage furnace operation.

The product from the first-stage operation was ground in a ball mill for about 2 hours. The ground product was analyzed and found to contain 4.03% carbon and 5.79% oxygen. Ground pellets from another first-stage batch containing more than the stoichiometric proportion of carbon to oxygen were added to the ground product to produce a stoichiometrical mixture and the mixture was then carefully blended. The mixture so prepared was then pelleted using a small amount of water as a binder and the pellets dried as in the first-stage treatment.

The dried pellets were then charged into a vacuum furnace. The pressure in the furnace was maintained at 2.5 mm. of mercury by incrementally increasing the furnace temperature to a maximum temperature of 1400" C. When the pressure dropped off at this temperature to a pressure level of about 75 microns of mercury, the.

furnace was cooled with argon to about 200 C. and discharged.

The final product was in the form of highly compacted,

uniformly porous, strongly cohesive, non-friable sintered pellets of chromium metal containing:

Percent Chromium 98.20 Iron 0.33 Silicon 0.20 Carbon 0.01 Manganese 0.01

- Sulphur 0.10. Oxygen 0.49

This application is in part a continuation of application Serial No. 101,240, filed June 24, 1949 and now abandoned.

What is claimed is:

1. Method of reducing chromium oxide by carbon which comprises preparing a mixture of chromic oxide and a carbonaceous reducing agent which is stoichiometric in carbon and oxygen with respect to the carbon monoxideforming reaction, pelleting said mixture, maintaining tilt: pellets so formed at a temperature of at least 1100* C. and below the melting point of any part of said pellets until the rate of evolution of carbon monoxide decreases markedly, cooling and comminuting the treated pellets, readjusting the composition of the comminuted material, if necessary, to obtain a stoichiometrical balance between carbon and oxygen, blending the comminuted material, pelleting the comminuted material, and maintaining the pellets so formed at a pressure less than atmospheric and at a temperature of at least 1100 C. and below the melting point of any part of said pellets until the rate of evolution of carbon monoxide decreases markedly.

2. A process as claimed in claim 1 wherein at least one of the materials in the group consisting of the oxides ide decreases markedly, cooling and comminuting the treated pellets, readjus'ting the composition of the comminuted material, if necessary, to obtain a stoichiometrical balance between carbon and oxygen, blending the comminuted material, pelleting the comminuted material, and maintaining the pellets so formed at a pressure less than atmospheric and at a temperature of between 1100" C. and 1600 C. until the rate of evolution of carbon monoxide decreases markedly.

4. Method of reducing chromic oxide by carbon which comprises preparing a mixture of chromic oxide and a carbonaceous reducing agent which is stoichiometric in carbon and oxygen with respect to the carbon monoxideforming reaction, pelleting said mixture, maintaining the pellets so formed at atmospheric pressure and at a temperature of between 1100 C. and 1600 C. until the rate of evolution of carbon monoxide decreases markedly,

cooling and comminuting the treatedpellets, readjusting the composition of the comminuted material, if necessary, to obtain a stoichiometrical balance between carbon and oxygen, blending the comminuted material, pelleting the comminuted material, and maintaining the pellets so formed at a pressure less than atmospheric and at a temperature of between 1100 C. and 1600 C. until the rate of evolution of carbon monoxide decreases markedly.

5. Method of reducing chromic oxide by carbon which comprises preparing a mixture of chromic oxide and a ide decreases markedly, cooling and comminutjng the treated pellets, readjusting the composition of the comminuted material, if necessary, to obtain a stoichiometrical balance between carbon and oxygen, blending the comminuted material, pelleting the comminuted material, and maintaining the pellets so formed at a pressure less than atmospheric and at a temperature of between 1250 C. and 1400 C. until the rate of evolution of carbon monoxide decreases markedly.

6. Method of reducing chromic oxide by carbon which comprises preparing a mixture of chromic oxide and a carbonaceous reducing agent which is stoichiometric in carbon and oxygen with respect to the carbon monoxideforming reaction, pelleting said mixture, maintaining the pellets so formed at a temperature of between 1250 C. and 1400 C. until the rate of evolution of carbon monoxide decreases markedly, cooling and comminuting the treated pellets, readjusting the composition of the comminuted material, if necessary, to obtain a stoichiornetrical balance between carbon and oxygen, blending the comminuted material, pelleting the comrninuted material, and maintaining the pellets so formed at a pressure of not more than 2.5 millimeters of mercury and at a temperature of between 1250 C. and 1400 C. until the rate of evolution of carbon monoxide decreases markedly.

References Cited in the file of this patent UNITED STATES PATENTS 2,205,386 Balke et a1. June 25, 1940 2,242,759 Schlecht et a1 May 20, 1941 2,470,790 Price May 24, 1949 2,541,153 Chadwick Feb. 13, 1951 2,606,110

Berge Aug. 5, 1952 

1. METHOD OF REDUCING CHROMIUM OXIDE BY CARBON WHICH COMPRISES PREPARING A MIXTURE OF CHROMIC OXIDE AND A CARBONACEOUS REDUCING AGENT WHICH IS STOICHIOMETRIC IN CARBON AND OXYGEN WITH RESPECT TO THE CARBON MONOXIDEFORMING REACTION, PELLETING SAID MIXTURE, MAINTAINING THE PELLETS SO FORMED AT A TEMPERATURE OF AT LEAST 1100*C. AND BELOW THE MELTING POINT OF ANY PART OF SAID PELLETS UNTIL THE RATE OF EVOLUTION OF CARBON MONOXIDE DECREASES MARKEDLY, COOLING AND COMMINUTING THE TREATED PELLETS, READJUSTING THE COMPOSITION OF THE COMMINUTED MATERIAL, IF NECESSARY, TO OBTAIN A STOICHIOMETRICAL BALANCE BETWEEN CARBON AND OXYGEN, BLENDING THE COMMINUTED MATERIAL, PELLETING THE COMMINUTED MATERIAL, AND MAINTAINING THE PELLETS SO FORMED AT A PRESSURE LESS THAN ATMOSPHERIC AND AT A TEMPERATURE OF AT LEAST 110*C. AND BELOW THE MELTING POINT OF ANY PART OF SAID PELLETS UNTIL THE RATE OF EVOLUTION OF CARBON MONOXIDE DECREASES MARKEDLY. 